President Trump signed an executive order to roll back environmental regulations put forth in the rule.

On Tuesday, President Trump signed an executive order targeting the Obama-era Clean Water Rule. The order directs the EPA and U.S. Army Corps of Engineers to review the rule, also known as the “Waters of the United States” (WOTUS) rule. But what does that mean, exactly? Here’s what you need to know.

What is the Clean Water Rule?

The Clean Water Rule, or Waters of the United States (WOTUS) rule, serves to clarify which streams and wetlands across the U.S. are protected under the Clean Water Act of 1972 and subject to federal regulations. The Clean Water Act made it illegal to discharge any pollutants into “navigable waters” without a permit, to ensure safe drinking water and habitats for humans and aquatic life. But the act didn’t actually define what “navigable waters” meant. And over time, questions arose about whether small streams and wetlands that feed into larger bodies of water, which serve as sources of drinking water, should qualify.

It was a tricky question for the Supreme Court, too, with the court splitting 4-1-4 on the case Rapanos v. United States, after the U.S. filed suit against real estate developer John Rapanos for illegally filling wetlands. Justice Anthony Kennedy’s opinion was that the Clean Water Act should apply to all wetlands that have a "significant nexus" to navigable waters (meaning, as interpreted later by the rule, that they have “an important and identifiable chemical, physical, or biological effect on downstream traditional navigable waters”), arguing that this applies even to those wetlands those that dry up occasionally or seasonally. Justice Antonin Scalia, on the other hand, argued that the regulations should only apply to wetlands with a “continuous surface connection” to a navigable water.

To put an end to the confusion, the Obama administration — particularly the EPA and the U.S. Army Corps of Engineers — set to work reviewing more than 1,200 scientific studies, which showed that smaller bodies of water needed protecting for the sake of the "health of larger downstream water bodies."

In 2015, they finalized the Clean Water Rule, which states that tributaries feeding into navigable waterways are automatically protected if they have a bed, a bank, and an ordinary high-water mark; as these are signs of flowing water and science has shown that they "can have a significant connection to downstream waters,” according to the EPA. The new rule also protects waters that are within 100 feet or a 100-year floodplain of nearby rivers, lakes, and their tributaries, as well as other specific waters (like California's vernal pools and Texas's coastal prairie wetlands) that have a “significant nexus” to navigable waters.

Why is it controversial?

Environmentalists praised the rule, which safeguards the water supply for the 117 million Americans (or one in three people) who get drinking water from previously vulnerable sources. Supporters also applauded the benefits — including financial ones — that clean water presents in reducing flooding, protecting wildlife habitats, and supporting hunting, fishing, and water recreation activities.

But critics, including farmers, fertilizer and pesticide makers, oil and gas producers, and golf course owners — and Republican lawmakers — argued that the move was a “power grab” and that it stifled their private property rights, which might be affected if they, for example, chose to block streams to create ponds for livestock or fill in wetlands to develop buildings, according to Politico.

According to NPR, after the 2015 rule, Owen McDonough of the National Association of Homebuilders said that the rule means a developer looking at purchasing rural land would “all of a sudden [be] faced with jurisdiction waters, and then he has to secure federal permits, and offset his impacts.”

And the American Farm Bureau Federation has argued against the rule, and the confusion it presents over whether they’d need permits for “any low spot where rainwater collects, including common farm ditches.”

But, the EPA notes, the Clean Water Rule (which only defines approximately 3 percent more waters than were previously protected), actually doesn’t regulate most ditches, nor does it change exemptions that were already in place for agriculture.

“I really think there’s a lot of bloodshed over this rule that didn’t need to be,” former EPA administrator Ellen Gilinsky told NPR. “I think there’s an opportunity to take another look at it and bring everyone to the table and get a rule everyone can feel good about.”

In the meantime, the rule is currently on hold, after a stay by the Sixth U.S. Circuit Court of Appeals.

What does Trump’s executive order do?

The executive order Trump signed Tuesday directs the EPA and the U.S. Army Corps of Engineers to review the rule, and ultimately propose a new rule to repeal or revise WOTUS. It also directs the EPA — led by administrator Scott Pruitt, who sued the EPA over this very regulation — to interpret “navigable waters” “in a manner consistent with” Justice Scalia’s Rapanos v. United States decision, when a new rule is proposed.

According to NPR, Trump reiterated concerns that the rule was a “massive power grab,” before singing the order on Tuesday. “Regulations and permits started treating our wonderful small farmers and small businesses as if they were a major industrial polluter,” he said. “They treated them horribly.”

Meanwhile, environmentalists reiterated their own concerns about doing away with WOTUS. “By attacking the Clean Water Rule and fundamental protections under the Clean Water Act, the President is putting the drinking water of 117 million people at risk, demonstrating that he puts the interests of corporate polluters above the public health,” Earthjustice president Trip Van Noppen said in a statement. “The action is a giveaway to polluters that will result in more contaminated waterways, more destroyed wetlands, and will harm small businesses and people in communities across the nation who rely on streams and wetlands for clean water.”

What happens next?

Getting rid of WOTUS isn’t as simple as signing an executive order. The rule was created through a lengthy legislative process, and it’ll have to go through another one to be repealed. “Repealing a rule requires a full public process and has to be justified by the law and the evidence available,” Jon Devine, an attorney with the Natural Resources Defense Council, told NPR. “And in the case of the clean water rule, that’s going to be rough sledding for the Trump administration.”

Indeed, in order to replace the existing rule, the EPA and U.S. Army Corps of Engineers would have to present enough scientific evidence, legal arguments, and public comments to prove the new proposal is a better option, and defend it all in court. Considering the Obama administration reviewed more than 1,200 scientific studies and put together a 400-plus-page technical report supporting the rule, the Trump administration is faced with a hefty task that could take years.

But even though the Trump administration may face an uphill battle, environmental organizations like Earthjustice are preparing for a fight. “Earthjustice will use the full strength of our nation's bedrock environmental laws to protect our communities and secure our future,” Van Noppen said in a statement. “If the new Environmental Protection Agency administrator Scott Pruitt and President Trump try to ignore the law, Earthjustice will continue to challenge them in court to ensure this Administration does not dismantle the basic mission of the EPA — the protection of our health and the environment.”